quinta-feira, 19 de fevereiro de 2015

UV: Exposure to sun poses risk of skin cancer even in the dark, study finds

Damage to skin cells continues for hours after spending time in the
sun, according to research that uncovers a new link between sun exposure
and cancer.
The discovery that some of the most serious damage to skin cells may
be occurring in the dark raises the prospect of new “evening after”
lotions that would help limit the effect and reduce the risk of skin
cancer.
Vitamin E was identified by scientists as a promising candidate for
helping to “quench” the after-effects of a hot day at the beach.
Unexpectedly, the source of the “dark damage” was found to be
melanin, the pigment in skin cells that normally acts as a shield
against ultra-violet (UV) radiation.
Prof Douglas Brash, who led the research at Yale University, said:
“If you look inside adult skin, melanin does act as a shield. But it is
doing both good and bad things.”
The latest work reveals that UV light produces a cascade of chemical
reactions, including the production of a “super bleach”, which reacts
with melanin causing one of its electrons to be “excited”. The melanin
then deposits its extra energy in the surrounding tissue. If a strand of
DNA happens to be nearby, it can absorb the energy causing the double
helix strand to bend, preventing the genetic code from being read
correctly.
“Melanin participates in the DNA-damaging part of this reaction only
for a few minutes, maybe only a few microseconds,” said Brash.

Exactly the same type of damage – called a cyclobutane dimer (CPD) –
is already known to occur directly during sun exposure as the UV rays
hit DNA strands and scramble the letters of the genetic code into
mutations. The more mutations skin cells accumulate over time, the
higher the likelihood that one of them will turn out to be cancerous.
However, until now, the damage was thought to stop as soon as a person took shelter in the shade.
The latest study, published in the journal Science, monitored skin
cells to reveal that mutations carried on appearing for about four hours
after a session of sun exposure equivalent to a “just perceptible”
sunburn.

When the scientists investigated further they discovered melanin’s crucial role in this process.
In the study, the scientists showed that potassium sorbate, a widely
used food preservative, was effective at blocking the ongoing damage,
although this isn’t “what you’d want to put on your skin”, according to
Brash. Vitamin E, which was also effective, would be a better candidate
and might explain why it is so effective in suncreams and moisturisers.
It was previously thought that Vitamin E simply worked by blocking UVB
rays.
Áine McCarthy, of Cancer
Research UK said, “The discovery that UV radiation can continue to harm
our DNA hours after exposure raises the possibility of developing
future products that might reduce this ‘dark damage’. For now, the best
way to cut your risk of skin cancer is to enjoy the sun safely and avoid
sunbeds.”
About 100,000 people are diagnosed with non-melanoma skin cancer
each year and about 13,500 with melanoma, which is rarer but causes far
more deaths.
The findings could help explain the different risk factors for the
two different types of skin cancer – melanoma is associated with brief
intense sun exposures, the kind that cause blistering sunburn, whereas
other skin cancers are normally linked to long-term moderate exposure.
It is possible that more “dark damage” may occur depending on whether
sunlight is absorbed over a longer time period.